This invention relates generally to recycling of hazardous waste materials, and more specifically, to systems and methods for waste pyrolysis, and utilization of waste materials as one or both of a fuel and a raw material in a cement kiln.
A cement kiln is an efficient tool for recycling hazardous waste and other waste materials. Typically liquid waste is used as fuel to provide energy in the cement making process. Solid and semisolid wastes, referred to collectively herein as solid wastes, are presently being incinerated or discarded in landfills. Incineration of solid wastes typically do not result in waste recycling and usually have poor to no energy recovery. Typically, ash from incineration is land filled or dumped into a river as liquid effluent.
However, such solid wastes have properties which allow for utilization as both a fuel and a raw material in cement making processes. Unfortunately simply firing a cement kiln with solid waste is difficult, since solid wastes typically contain larger fractions of metals and inorganic materials than does liquid waste. For a cement making process it is desirable to have a fuel/raw material stream that is homogeneous, can be safely stored in large silos or bins, is easy to transport, can provide a reliable raw material component for making cement, and does not contain significant amounts of metals.
It is also desirable to provide material in a physical form that does not cause reduced cement clinker quality. For example, when shredded solid waste fuel is transported into a clinkering zone of a cement kiln, chunks of carbonaceous waste material too large or heavy to instantly burn, fall into the hot cement clinker. The chunks of waste material can cause localized oxygen deficiency and therefore reduced clinker quality. It is desired that the waste derived feed/fuel be provided as a finely divided uniform powder. Unfortunately solid waste available from a wide range of industrial processes is not provided in such a form. Solid waste is not uniform, it can not be safely stored in any large vessel or pile, and it is very difficult to transport. Most solid waste further contains large and largely varying metal content, and is not a reliable raw material for cement manufacture due to its highly variable composition.
Several methods are available for processing solid waste fuels so that they can be used in a cement kiln. The methods include shredding, grinding, or high intensity mixing with liquid fuel. Unfortunately none of these methods provide a waste fuel which satisfies the above described waste fuel qualities. High intensity mixing with liquid waste allows a portion of the solid waste to be slurried so that it can be stored in slurry tanks and homogenized, but the process has many shortcomings. Metals in the solid waste lead to high wear, high maintenance, down time, and high labor costs, and when removed from the mixer, are not sufficiently clean to be recycled without further processing and expense. Other very common solid waste components such as plastics and rubber materials cannot be processed in the liquid mixer system. Shredding and grinding are dangerous high labor and high maintenance operations, which result in hazardous non-homogenous mixtures that can only be safely stored in small piles and/or be kept under inert atmospheres and remain very difficult to transport. Due to the non-uniformity and difficulty in handling, only a small fraction of a total cement kiln fuel can be prepared by shredding or grinding.
Conventional waste preprocessing methods do not adequately separate metals from cement kiln bound waste. Since many of the metal objects found in the waste materials are usually attached to rubber, plastics, or other inorganic materials, the recovered metals stream in conventional waste preprocessing methods needs to be further cleaned or separated before being recycled. Furthermore, the process of shredding causes the metal objects to roll around and encapsulate the non-metal components making them difficult to separate. In addition, aluminum, copper, brass, stainless steel and other non-magnetic objects cannot be recovered by this conventional method. For example, if drums which contain printed circuit boards, plastic insulated copper wire, and partly solidified rubber are shredded, and then sent to a cement kiln, all of the metals in the waste are put into the kiln. Therefore, lead, copper and steel parts are not recovered. In addition, steel cannot be magnetically recovered after the difficult and expensive shredding operation, since it is encrusted with sticky rubber and fiberglass. Metals such as lead and copper cannot be recovered at all. If this waste was processed by gasification or high temperature pyrolysis the volatile metals such as lead would be vaporized and would not be recovered.
Thermal processing techniques have been proposed to process solid waste such as gasification and pyrolysis. Gasification processes for waste are utilized to produce fuels for other processes, while metals and other inorganic waste materials are disposed of in other forms, which are thought to be more environmentally acceptable than incineration processes. Gasification is a process where oxygen-bearing material is added to the waste to convert some or substantially all of the carbonaceous material within the waste to a gas. All of the carbonaceous fuel is converted to a gas during the gasification process. Such gas is not practical to store and has a low quality rating, typically 4 to 14 MJ/Nm3. Therefore, waste fuel from the gasifier cannot be safely stored in a uniform manner, and metals exiting the gasifier cannot be recycled because they are trapped in an inorganic slag stream. Since the raw solid fuel entering the gasifier is highly variable, quality of the produced gas is also highly variable and therefore requires more complex burner control systems. Other problems with the gasification processes are that volatile metals such as lead cannot be recovered. In addition, the gasification process requires temperatures between 1000xc2x0 C. to 1400xc2x0 C., which are not available in a cement making process. Further, the high temperatures require special materials of construction that result in high maintenance, high operating costs, potentially dangerous operation, and potential unreliability. Finally, solid waste materials have to be removed from the drums and pre-processed before gasification.
Solid waste is pyrolyzed when it is heated above 400xc2x0 C. in a low oxygen atmosphere. Commercial pyrolysis is normally carried out between 400xc2x0 C. and 800xc2x0 C. This results in a pyrolysis gas stream and a solids stream. The ratio of gas to solid pyrolysis product of a given material is primarily dependent on the heating rate and temperature in the pyrolysis reactor. Most pyrolysis reactors are designed to minimize the solid fuel and to produce a low tar gaseous fuel that can be burned in engine driven generators or boilers. Reducing the tar content of the gaseous fuel results in a reduction in the energy density of the gaseous fuel. Solids resulting from these processes are typically hauled to a landfill at significant cost. Pyrolyzing at a lower temperature for a longer time results in the largest yield of solid fuel and results in the highest fuel gas energy density, typically 14 to 22 MJ/Nm3.
Most known pyrolysis processes burn a majority of the resulting pyrolysis gas to provide heat for continuing the pyrolysis process. Therefore, less of the heat energy from the waste is available for other uses. Further, a flue gas is created from such burning, which must be scrubbed or treated at still additional cost. Directing such flue gas into, for example, a cement kiln for scrubbing is very undesirable since it reduces the cement kiln capacity and increases cement kiln dust loss. Known pyrolysis apparatus further require drums of solid waste to be shredded, sorted, or separated prior to entering the pyrolysis reactor, since objects in the waste can cause jamming, breakage or clogging. For example, drums of waste can contain large metallic objects such as crankshafts, wrenches, or even four inch thick lead disks 23 inches in diameter. Pre-processing of the drums of waste result in increased machinery and processing cost, and results in more human exposure to the waste. In many instances it is extremely difficult to remove certain wastes from the drums. For example, when liquid polymer or rubber wastes are poured into a drum then solidified, walls of the drum are bonded to the waste. Additionally pre-processing often removes difficult to process materials such as plastic, paint, resin, or rubber adhered metals that would otherwise benefit most from the thermal separation capability of the pyrolysis process. While many pyrolysis processes have been developed, none of these are ideally suited to produce cement kiln fuel/raw material.
In one aspect, a method for pyrolyzing waste materials in a pyrolysis system which utilizes waste heat from a cement kiln is provided. The pyrolysis system includes a feed inerting section and a pyrolysis chamber. The provided method comprises feeding drums of waste into the feed inerting section, replacing oxygen in the feed inerting section with carbon dioxide recovered from the cement kiln, and transporting the drums of waste through the pyrolysis chamber. Further, the method comprises pyrolyzing the waste in the pyrolysis chamber with exhaust gasses from the cement kiln and routing a fuel gas created by the pyrolysis to provide heat to the cement kiln.
In another aspect, a waste pyrolysis system for drums of waste is provided. The system comprises a cement kiln, a feed inerting section, a pyrolysis chamber heated by exhaust gasses from the cement kiln and creating a pyrolysis fuel gas utilized for heating the cement kiln. The system also comprises an exit inerting section and a drum unloading section.
In still another aspect, a pyrolysis chamber is provided which comprises a heat transfer source which utilizes exhaust gases from a cement kiln to externally heat the chamber and an exhaust for pyrolysis gases, the exhaust configured to provide heat to the cement kiln.
In yet another aspect, a pyrolysis chamber is provided which comprises a rectangular metallic duct, at least one hot air jacket section surrounding the metallic duct, and a plurality of baffles. Each baffle has an opening and is attached to the jacket section and configured to provide support for the metallic duct. The baffles are arranged such that the openings alternate between a top and a bottom of the metallic duct.